Today: Radioactivity Introduction

Some of our goals for the next few lectures

Learn about the physics of radioactivity and nuclear reactions.

Learn about the hazards and safety mechanisms

Be able to apply the physics to real-life debates related to nuclear physics

A soap bubble or film is a thin layer of water stabilized by detergent molecules

http://www.funsci.com/fun3_en/exper2/exper2.htm

Water: speed of light slower

Air: speed of light fast

Air

Incident light will reflect and pass throughinterfaces.

Reflections off the top surface and bottom surface will interfere with each other

KEY POINT: 180 degree phase shift when reflecting off interface going from fast to slow (top interface in this case)

Detergent layer

180 degree phase shift explains the black color in the thinnest part of the soap film

Thinnest part of film

http://www.funsci.com/fun3_en/exper2/exper2.htm

Water: speed of light slower

Air: speed of light fast

Air

When film is very thin, path length difference is negligible. So interference is destructive.

Appears black because I used black mug.

As film gets thicker, there is some path length difference, so interference is not completely destructive – white color.

Wikipedia.org

Detergent layer

Rachelle Regan

Jocelle B.

Rachelle: “I do not agree with the book because when I look at my picture of my bubbles I do see a lot of green”

John Z.: “I will say that the predominant colors will be the green and magenta. Yellow and cyan dot appear as much.”

Laura F.: “I saw greens, blues, reds, and violets. I think the author should explain how varied angles of light striking the bubble can have an effect on color. “

(click on movie)

If we were to learn and apply the mathematical physics,we could very well predict the pattern of colors!

You know which colors are missing / present (e.g. magenta = blue + red)It is a really nice way to apply your knowledge of physics!

Today: Radioactivity Introduction

Some of our goals for the next few lectures

Learn about the physics of radioactivity and nuclear reactions.

Learn about the hazards and safety mechanisms

Be able to apply the physics to real-life debates related to nuclear physics

First! Cloud Chamber Demo

We will see the trails of particles from radioactive decay!

Andrew Foland / Cornell University / Wilson Lab /

Slide Projector

Alcohol-soaked side

Dry Ice

Super-saturated alcohol vapor

Alpha particlesource

Let’s observe the demo first, and then learn more about what we saw.

http://www.youtube.com/watch?v=chxv5G6UFl0&feature=relatedYou can build one yourself!

One slide about charge (electrical charge)

Particles in the universe can have positive (+), negative (-) or neutral (zero) charge

Opposite charges attract each other Same charges repel each other

Force Charge1 * Charge2

distance2

“is proportional to”

Physics can exactly predict the force between two charged objects:

Increase charge = increase forceIncrease separation = decrease force

The main reason I wanted to show you this slide:Charge is conserved; Charge is quantized (+1, +2, etc. -1, -2, etc.)

“Conservation of Charge” … akin to “conservation of energy”

Clicker Question—Conservation of charge

Only one of these hypothetical events is allowed by the law of “conservation of charge.” Which one?

A) Two electrons (-1 charge each) combine to form a neutron (0 charge)

B) Two protons (+1 charge each) annihilate with two neutrons to form four gamma ray photons (0 charge)

C) An electron and a positron (+1 charge) annihilate to form two gamma rays (0 charge)

Clicker Question—Conservation of charge

Only one of these hypothetical events is allowed by the law of “conservation of charge.” Which one?

A) Two electrons (-1 charge each) combine to form a neutron (0 charge)

B) Two protons (+1 charge each) annihilate with two neutrons to form four gamma ray photons (0 charge)

C) An electron and a positron (+1 charge) annihilate to form two gamma rays (0 charge) This actually happens!

OK, one more thing about charge

Charged particles move in curved pathswhen in a magnetic fieldNeutral particles move in straight paths

Path of a charged particle

“B” is magnetic field

Summary of charge characteristics

Particles in the universe can have positive (+), negative (-) or neutral (zero) charge

Opposite charges attract each other Same charges repel each other

Force Charge1 * Charge2

distance2

“is proportional to”

Charge is conservedCharge is quantized (+1, +2, etc. -1, -2, etc.)

Charged particles move in curved pathswhen in a magnetic fieldNeutral particles move in straight paths

Path of a charged particle

Refresher on the atom

Atoms are composed of electrons (-1), protons (+1) and neutrons (zero)

The type of element is determined by the number of protons.

6 proton = carbon; 7 protons = nitrogen; 8 protons = oxygen

Two atoms with same number of protons, but with differing number of neutrons are different isotopes of the same element

Helium Atom

Proton

Neutron

Electron “Cloud”

Refresher on the atom

Because the electron “clouds” surround the nucleus, most interactions are governed by the physics of electron-electron interactionsi.e. the electrical force … “Electronic reactions” = Chemistry

The electrons have been involved in most of what we’ve been talking about: atomic spectra; fluorescence; interaction of matter and electromagnetic waves

When an atom loses or gains an electron, it is still the same element!(It is called an ion)

Helium Atom

Proton

Neutron

Electron “Cloud”

X-rays discovered in 1895, Wilhelm Röntgen

Cathode Ray Tube (CRT) Röntgen noticed fluorescence across the room (through books and papers)

While experimenting with putting objects in the unknown beam, he saw images of his (wife’s) bones appear!

Shortly thereafter, radioactivity discovered by Henri Becquerel (1896) (1903 Nobel prize with the Curies)

Uranium Salt

Photographic film

Expose to sunlight

Cloudy day /Dark drawer for a couple days

Much, much larger effect!

The uranium itself was emitting radiation!

This sparked the exciting ageof nuclear physics, led by Rutherford

Clicker question—Invention of Radioactivity

When did Physicists invent radioactivity?A) ~1900B) World War IC) World War IID) None of the above

Clicker question—Invention of Radioactivity

When did Physicists invent radioactivity?A) ~1900B) World War IC) World War IID) None of the above

Radioactivity has existed since the infancy of the universe!

For example, the uranium that Roentgen used was naturally occurring.Uranium is about as abundant as arsenic.

Molecules in the atmosphere are radioactive.

*** The interior of the Earth is heated by radioactive decay!

Three kinds of radiation emitted via radioactive decaynamed before they were understood…alpha, beta, gamma

Alpha particles stopped by paperalpha particles are helium nuclei

Beta particles stopped by aluminumbeta particles are electrons (or positrons)

Gamma rays stopped by leadgamma rays are high energy photonsx-rays similar, but lower energy than gamma

Clicker question…charge of radiation

Let’s say by examination, physicists and chemists discovered that one type of the radiation was not deflected by magnetic fields. Which of the following would be true?

A) The un-deflected radiation must be charged.B) The un-deflected radiation must be uncharged.

Clicker question…charge of radiation

Let’s say by examination, physicists and chemists discovered that one type of the radiation was not deflected by magnetic fields. Which of the following would be true?

A) The un-deflected radiation must be charged.B) The un-deflected radiation must be uncharged.

Charged particles travel in curved paths when in magnetic fields!

Path of a charged particle

Alpha, beta and gamma radiation also distinguished by charge behavior

Alpha particles deflected by magnet

Gamma rays unperturbed by magnet

Beta particles deflected opposite alpha

It was noticed early on that beta particles behaved exactly like cathode rays (electrons)Alpha particles were discovered to be helium nucleiGamma rays were very high energy x-rays (photons)

Explanation of Geiger counter

Depending on the design:

Maybe detect alpha-radiationCAN detect beta-radiationMaybe detect gamma-radiation

Alpha particles difficulty passing through windowBut can make it through mica

Ionizing radiation (beta, gamma, some alpha)enter through window and create ions inside.

These events are detected via the electric current and turned into audible “clicks.”

OK, Let’s try to reinforce some learning about alpha, beta, gamma radiation

http://www.youtube.com/watch?v=NP9Woxbkr_M

The “Inverse-Square Law”

First, an introduction to the concept using bread and a butter gunhttp://www.youtube.com/watch?v=JW3tT0L2gpc

The intensity of radiation follows the inverse square law

Force Charge1 * Charge2

distance2

Intensity of radiation Source intensity

distance2

Remember for electric charge:

If you double the distance,Intensity decreases by a factor of 4

Let’s explore the inverse square law with a radiation demo

We should be able to make predictions for how intensity of radiation will change.

Also can investigate properties of shielding

Geiger counterMoveable shielding

Radioactive source?